Manufacturing methods based on organic semiconductor materials resulting in attractive applications like Organic Light Emitting Devices (OLEDs), also known as Organic Electroluminescent Devices (OELs), and photovoltaic devices (i.e. organic solar cells) are of great interest today. The main advantages of using organic materials are low cost and a capacity for large-area deposition, even on flexible substrates. Furthermore, the large variability of organic compounds allows tailoring the materials for specific applications.
The OLED is a light-emitting device that typically consists of a number of organic layers which are based on small molecules and/or polymers and sandwiched between two electrodes, the anode and the cathode. Each layer is optimized for its own functionality. The light emitting area of the organic light emitting device preferably consists of a hole transport layer, an emitting layer, and an electron transport layer. The light-emitting is based on electrodes with electrons and holes injected from the cathode and the anode that are used to selectively excite levels in the organic molecules of the organic layers.
Unlike Liquid Crystal Displays (LCD) and field emission displays (FED), which are constructed of separate layers of materials that have been assembled, OLEDs are monolithic devices, because the layers are deposited on each other, creating a single unit.
The photovoltaic device has the fundamental structure of an OLED. The main difference is that in these devices a photovoltaic mode of the organic layers is utilized such that shining a light on the device results in a current and/or voltage which is generated at the electrodes (in the OLED a light emitting mode is utilized such that application of a voltage across the electrodes causes light to be emitted). In the following mainly OLEDs are described, but the same principles yield for organic solar cells.
For large area OLED lighting, a large current is required to drive the device. For bottom emissive small area OLEDs the cathode usually has a low enough resistance, however for large area lighting applications the resistance has to be at least 10 times lower. Using common thin film anode and cathode materials, e.g. ITO and Al respectively, results in a large sheet resistance and the large currents give rise to substantial voltage drop. Examples of voltage drops caused by the anode and cathode for different electrode sheet resistance are shown in Table 1.
TABLE 1Voltage drop [V]Voltage drop [V]SubstrateIlluminationLuminationΩ per square20 lm/W100 lm/W20 lm/W100 lm/W1522545225451016032160321163.2163.20.11.60.31.60.30.010.160.030.160.03
The voltage drop gives rise to inhomogeneous luminance of the large area. The sheet resistance of the anode and cathode metals sets a limit to the maximum size of a uniformly lit area, a light tile, which has an area in the order of a few square centimeter in the current material systems. For large area applications the sheet resistance of the metal should be well below 0.01Ω per square.
Techniques to further increase the area of the light tile are known. In these techniques additional metallization is added onto the substrate to decrease the sheet resistance. Referring to FIG. 1b, which illustrates a prior art light tile, a large area tile 40 is subdivided into subtiles, or pixels, 44 for which extra fine metallization must be used, which metallization pattern is hereinafter referred to as a mash 42. The subtiles 44 are interconnected by good conducting metal tracks, which tracks are herein after referred to as a grid 41. The additional metallization acts as a shunt and provides an overall lower sheet resistance. For display purposes the anode requires extra metallization for shunting according to the description above.
For the implementation of the extra metallization for shunting in an OLED two different technologies are currently used to perform the two types of patterned metallization: 1) For the mash manufacture thin film technology is used. This becomes rather expensive due to the use of photolithography and long deposition times in expensive machines. 2) For the grid manufacture thick film technology is used, which adds extra fabrication steps in the manufacturing of the display. Thus, the manufacture of the patterned metallizations is associated with drawbacks of being complicated and expensive.